Touch-screens are used in conjunction with a variety of display types, including cathode ray tubes (i.e., CRTs) and liquid crystal display screens (i.e., LCD screens), as a means of inputting information into a computer system. When placed over a display, the touch-screen allows a user to select a displayed icon or element by touching the screen in a location that corresponds to the desired icon or element.
A variety of touch-screen types have been developed. One type of touch screen utilizes transparent layers of resistive material separated by a pattern of insulative material. When a user presses on the touch screen, the layers of resistive material come into contact with one another and complete a circuit. By utilizing a voltage gradient in the circuit, the magnitude of the voltage at the point of screen compression can be used to determine the compression location along one axis of the screen. The use of this technique along two orthogonal axes provides the actual location at which the screen was pressed.
There are two types of resistive touch-screens dominating the market today: four-wire systems and five-wire systems. Four-wire systems have had more commercial success than five-wire systems, primarily due to their low power consumption and the simplicity of the required external circuitry. However, five-wire touch-screens are preferable from a reliability standpoint as these touch-screens are typically rated to survive at least an order of magnitude more touches than four-wire systems. The difference in reliability is due to the basic differences in screen design. In a four-wire system the top resistive layer is used to measure directional current along one axis. Therefore the conductivity of the top resistive layer must remain uniform. However, as the top surface undergoes repeated compressions, the uniformity of the resistive layer gradually degrades, leading to inaccurate readings and eventual screen failure. In contrast, all directional measurement in a five-wire system is provided by the lower resistive layer. The upper surface must merely retain its conductivity. Therefore changes in the uniformity of the resistive layer of the top surface does not degrade the performance of the five-wire system until this layer undergoes complete failure (i.e., all conductive pathways are lost).
There is an increasing trend to incorporate touch technology into a wide variety of system applications, and a new generation of microprocessor components and system software designed to serve this new market is becoming commercially available. The initial market for which these components and software were designed employed a four-wire touch screen and consequently, it is this four-wire interface that predominates. Further, there is a large customer base in the process control and medical segments of the touch-screen market that employs custom electronics and software designed to interface with four wire systems, which, by reason of their shorter life cycle, must be replaced on a regular basis. Therefore, if an end-user wishes to incorporate a five wire touch-screen into his design, or to replace a four wire touch-screen with the more reliable five wire unit, new interface electronics and software are necessary, the design of which constitutes an economic barrier to accessing these growing market segments.
From the foregoing it is apparent that a converter that would allow a five-wire touch-screen to be plugged into a four-wire controller without modifying the controller hardware and software is desirable.